An optical transport network, OTN, comprises optical network elements connected by optical fiber links. An optical transport network provides transport, multiplexing, switching, management and supervision of optical trails carrying client data. Optical transport networks have been migrating from SONET technology to a wavelength division multiplexing, WDM, architecture which allows to carry multiple wavelengths over a single fiber.
Traffic engineering, TE, is the process of selecting a path for routing a given connection from a source network element to a destination network element such that the selected path satisfies constraints of the given connection by simultaneously optimizing the network resource utilization and avoiding traffic congestion. Traffic engineering on a transport network such as an OTN/ODUk layer network presents many challenges. One such challenge is the complexity of managing network elements that make up the transport network. A further challenge for traffic engineering is the amount of information about network elements and links interconnecting them that needs to be constantly rediscovered and redistributed to traffic engineering applications such as path computers in real time. As a consequence of the high amount of information and data, the necessary size of traffic engineering databases increases and therefore memory and computation power requirements are high, especially considering the complexity of required path computation algorithms. Moreover, the service paths often need to be computed in real time, in particular in the context of applications such as network failure service recovery. The growth in size of the network increases the challenges for traffic engineering exponentially.
Packet-switched layer networks can be considered as built of symmetrical non-blocking switches. Such a symmetrical non-blocking switch can receive a service traffic over one of its interfaces and send it out over any other its interface with no limitations and with the same effect on the respective quality of service such as packet loss ratio or delay. Path computation and traffic engineering on networks made of interconnected symmetrical non-blocking switches are relatively simple tasks. Specifically, there is no traffic engineering information required to be advertised on per network element basis, and there are only few simple attributes that need to be advertised on per link basis. These attributes can include for instance unreserved bandwidth and TE metrics. Furthermore, the advertisements need neither to be accurate nor frequent. The employed path computation algorithms are relatively simple and can be performed on a roughly up-to-date traffic engineering database, TED.
In contrast, network elements making up a circuit-switched transport network often exhibit switching limitations and hence, generally speaking, must be considered as blocking switches. A fixed optical add-drop multiplexer (FOADM) in a WDM layer or cascaded ADM in a OTN/ODUk layer network, when receiving a service traffic on one of its interfaces, often can switch it onto a subset of It's interfaces, but not to any other interface.
This circumstance complicates traffic engineering in two ways. First, information about switchable interface combinations of each network element needs to be made known to the path computer or otherwise nothing can prevent the path computation process to select non-provisional paths. This is usually solved by having network elements advertise their interface connectivity matrices explicitly describing valid and/or invalid interface switching combinations. Second, path computation algorithms need to be adjusted accordingly to deal with the advertised network element interface connectivity matrices.
Traffic engineering gets even more complex when the abovementioned switching limitations exhibited by a given network element happen not on an interface level, but on individual atomic resource level. Specifically, it is possible that a given wavelength channel on a given inbound WDM layer FOADM interface can be switched onto one set of the FOADM's outbound interfaces, while another wavelength channel, belonging to the same inbound interface, can be switched onto a different set of the FOADM's outbound interfaces. Likewise, it can be possible that a given ODUk container on a given inbound OTN cascade's interface can be switched onto one set of the cascade's outbound interfaces, while another ODUk container, belonging to the same inbound interface, can be switched onto a different set of the cascade's outbound interfaces.
This individual resource switching limitation can be overcome by advertising every individual resource separately with an explicit specification of the resource's switching capabilities as one of the resource's advertised attributes. Such a conventional approach can make sense in WDM layer networks, however is not applicable to OTN/ODUk layer networks. The reason for this is that WDM layer atomic resources (wavelength channels) of a WDM layer network are not hierarchical in nature (wavelength channels cannot carry other wavelength channels), and the number of wavelength channels per interface is relatively small. Further, in a WDM layer network, there are other reasons as to why it is important to advertise each wavelength channel individually. For instance, this is done to honor wavelength continuity constraints and to be able to compute wavelength specific optical impairments constraints.
In contrast, in an OTN/ODUk layer network, the individual resources are hierarchical. For example, an ODU2 container can carry ODU1 containers, which in turn can carry ODU0 containers. There can be potentially a big number of ODUk containers associated with a given interface of the network element. Consequently, advertising each container individually is likely to cause grave scalability issues. Furthermore, if not for the switching limitations, there is no reason to advertise each container individually. Instead, just relatively infrequent advertising of rough total numbers of available containers on per container type basis would suffice for path computation.
Accordingly, there is a need for a method and apparatus for traffic engineering on an optical transport network which allows to honor individual ODUk containers switching limitations without the necessity of advertising each ODUk container on every OTN/ODUk link individually.